Process for the reduction of metal oxides



United States Patent 3,386,817 PROCESS FOR THE REDUCTION OF METAL OXIDESGilbert S. Layne and James 0. Huml, Midland, Mich., and Richard D.Smith, Madison, Conn., assignors to The Dow Chemical Company, Midland,Mich., a corporation of Delaware No Drawing. Filed Sept. 10, 1965, Ser.No. 486,556 6 Claims. (Cl. 75-84) This invention relates to an improvedprocess for the reduction of metal oxides and more particularly relatesto a process for the thermal reduction of metal Oxides by a metalreducing agent in the presence of molten A1 It is known to reduce metaloxides to the corresponding metal with aluminum at elevatedtemperatures. The known process generally involves heating finelydivided aluminum and a metal oxide to produce the metal plus A1 0 Such aprocess, however, must be conducted in relatively small batches due tothe rapid heat evolution involved. Also, the reaction generally will notgo to completion as the A1 0 tends to coat the metal oxide beingreduced. Separation of the metal from the A1 0 is difficult due tosintering and the use of various metal halide fluxes has met with onlyvery limited success.

It is an object of this invention to provide an improved process for thereduction of metal oxides. It is a further object of this invention toprovide a process for the thermal reduction of metal oxides with a metalreducing agent which produces a relatively pure metal product, rapidly,in good yields and in an easily recoverable form. It is a further objectof this invention to provide a process for the reduction of metal oxideswhich may be conducted continuously or in relatively large batches.These and other objects and advantages of the process will be readilyappreciated and better understood by reference to the following detaileddescription.

It has now been discovered that metal oxides may be reduced to the metalthereof by the reaction of such metal oxide with a reducing agentselected from the group consisting of aluminum, silicon, calcium andmagnesium at elevated temperature in the presence of A1 8 as a solventor diluent for the reaction system.

Metal oxides which are subject to reduction in the process of thisinvention include the oxides of titanium, zirconium, hafnium, tantalum,chromium, throium, vanadium and the like. The metal oxide to be reducedmust, of course, be non-volatile at the melting point of A1 8 and itssulfide must be less thermodynamically stable than A1 5 It has beensurprisingly discovered, however, that in this process the metal oxideto be reduced need not be less thermally, or thermodynamically, stablethan the oxide of the reducing metal. The product of such reaction isgenerally the intermetallic compound or alloy of the reducing agent andthe product metal.

As metallic reducing agents, aluminum, silicon, calcium or magnesium maybe employed. The aluminum is particularly advantageous, however. Inorder to achieve the advantages of this process, it is necessary thatthe reduction be carried out in the presence of liquid Al S SufficientA1 8 must be maintained in the reaction zone to dissolve the aluminumoxide or other oxides produced by the reduction. Additional A1 8 may beused, however, and the use of such excess is frequently desirable toassure a low viscosity reaction system.

When aluminum metal is used as the reducing agent, A1 8 serves as asolvent for the metal oxide to be reduced, a partial solvent for thealuminum and a solvent or complexing agent for the A1 0 produced. SinceAl S is at least a partial solvent for the reactants, molecular contactis made between such reactants which produces rapid and completeconversion of the metal oxide to the free metal. Usually a reaction timeof from 30 minutes to 2 Patented June 4, 1968 hours is sufiicient toassure complete reduction of the metal oxide. Longer contact times arenot generally detrimental, however.

When silicon, calcium or magnesium are employed as reducing agents, itis thought that an intermediate reaction occurs with the A1 8 to formthe sulfide of the reducing agent and consequent release of aluminummetal. The aluminum then reduces the metal oxide. Sullicient A1 5 mustbe present, therefore, to both react with the reducing agent and todissolve the A1 0 present. The CaS or MgS thus formed dissolves in theA1 8 and may be separated as products. The SiS thus formed will leavethe reaction zone as a gas. It is known that CaS and MgS are formed whentheir respective metals are used as reducing agents herein but theactual mechanism is not known with certainly and this invention is notto be construed as limited thereto.

The reaction temperature must be above the melting point of A1 5 andbelow its boiling point at the pressure employed. At atmosphericpressure, therefore, the reaction temperature may be between about 1075C. and about 2100 C. Such temperatures may be varied by varying thepressure on the reaction system but pressure is not a critical factor inthis process and atmospheric pressure is generally the most convenient.Since relatively high temperatures are involved in this process andmolten metals are exposed to the atmosphere in the reaction zone, it isdesirable that such atmosphere be chemically inert to the reactants andproducts. An atmosphere of an inert gas such as argon has been found tobe suitable.

If the oxide to be reduced is a solid at reaction temperature, it isdesirable that such oxide be in a finely comminuted form to increase itsrate of dissolution in A1 8 The use of A1 5 as a solvent and diluent inthe reaction system likewise makes it possible to control the particlesize of the metal product by varying the residence time and temperatureor by controlling the cooling rate of the reaction mass. For example,flakes of titanium maintained at 1800 C. for 2 hours produces flakes ofabout 1 mm. in size whereas the same material held at 1600-1650 C. for 1hour produces flakes of only about one-fourth mm.

In order to provide ease in understanding, the following examples areset forth to illustrate the invention but are not to be construed aslimiting to the scope thereof.

Example 1 A mixture of 120 g. of TiO 54 g. of Al and 100 g. of Al s; ina graphite crucible was heated to 1800 C. at atmospheric pressure fortwo hours by induction heating. The reaction zone was maintained in anargon atmosphere. At the end of the reaction period, the melt wasallowed to cool to room temperature, leached with water and analyzed.Analysis of the water leached product showed it to contain A1 0 Al(OH)and titanium metal powder. No TiO: was present. Separation of A1 0 andAl(OH) from Ti metal is easily accomplished by using the water overflowtechnique whereby the A1 0 and Al(OH) will be carried out by the flowingstream of water and the heavier titanium metal particles will remainbehind.

Example 2 A mixture of 40 g. TiO 42 g. A] and g. A1 8 was heated in agraphite crucible to 1600 C. at atmospheric pressure for 1 hour byinduction heating. The reaction zone was maintained in an argonatmosphere. At the end of the reaction period, the melt was cooled toroom temperature and leached with water to produce a residue of metallicflakes and oxide powder. The powder was washed away by water overflowand the metal flakes collected. Analysis of the metal showed them tocontain equal parts of aluminum and titanium.

3 Example 3 A mixture of 61.5 g. ZrO 63.5 g. Al and 75 g. A1 8 washeated in a graphite crucible to 1700 C. at atmospheric pressure for 1hour by induction heating. The reaction zone was maintained in an argonatmosphere throughout the reaction. At the end of the reaction period,the melt was cooled to room temperature, leached with water and theproducts were analyzed. Analysis of the solid product after waterleaching showed it to contain metal flakes and a white powdery oxide.The flakes were washed free of the oxide and analysis showed them tocontain Al and Zr in the ratio of the compound ZrAl X-ray diffractiondata confirmed this identification.

We claim:

1. A process for the reduction of refractory metal oxides whichcomprises admixing said metal oxide with A1 8 and a reducing agentselected from the group consisting of aluminum, magnesium, calcium, andsilicon at a temperature between the melting point and the boiling pointof A1 5 at the pressure employed for a period sufficient to affect suchreduction.

2. A process for the reduction of refractory metal oxides to thecorresponding metal wherein said metal oxide is non-volatile at themelting point of A1 8 and the sulfide of the metal to be produced isless thermodynamically stable than A1 8 which process comprises admixingsaid metal oxide with A1 5 and a reducing agent selected from the groupconsisting of aluminum, magnesium, calcium and silicon at a temperaturebetween the melting point and the boiling point of A1 8 at the reactionpressure employed for a period sufiicient to afiect such reduction.

3. The process of claim 2 wherein the metal oxide is T102.

4. The process of claim 2 wherein the metal oxide is ZIOZ.

5. The process of claim 2 wherein the reducing agent is aluminum.

6. A process for the reduction of TiO to metallic titanium whichcomprises admixing said TiO with A1 5 and metallic aluminum at atemperature of between 1075 and 2100 C. and at substantially atmosphericpressure for a period of from 30 minutes to 2 hours.

References Cited UNITED STATES PATENTS 2,995,439 8/1961 Litz 75-843,047,477 7/1962 Spraul et al. 75-84 X 3,099,555 7/1963 Teitel 7584.l3,184,302 5/1965 Chindgren 7584 3,288,593 11/1966 Smith et al 75843,288,594 11/1966 Smith 75-84 CARL D. QUARFORTH, Primary Examiner.

M. J. SCOLNICK, Assistant Examiner.

1. A PROCESS FOR THE REDUCTION OF REFRACTORY METAL OXIDES WHICHCOMPRISES ADMIXING SAID METAL OXIDE WITH AL2S3 AND A REDUCING AGENTSELECTED FROM THE GROUP CONSISTING OF ALUMINUM, MAGNESIUM, CALCIUM, ANDSILICON AT A TEMPERATURE BETWEEN THE MELTING POINT AND THE BOILING POINTOF AL2S3 AT THE PRESSURE EMPLOYED FOR A PERIOD SUFFICIENT TO AFFECT SUCHREDUCTION.